Susi

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- [ADDED] Quantization error `get_quantization_error()`

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- [ADDED] Landing page with vuepress.
- [ADDED] Conda-forge recipe.
- [ADDED] Function `SOMClassifier.predict_proba()`
- [ADDED] Example notebook for multi-output regression
- [CHANGED] Code formatting to black.
- [CHANGED] CI from travis to GitHub actions.
- [FIXED] Requirements in setup.py

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- [ADDED] Python 3.9 support. Python 3.6 support will be removed soon.
- [CHANGED] Function names for private use now start with an underscore.

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- [ADDED] New distance metric "spectralangle".
- [ADDED] FAQs.
- [ADDED] Separate between positional and keyword parameters.
- [ADDED] Plot script for neighborhood distance weight matrix.
- [FIXED] Added inherited members to code documentation.

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- [ADDED] Logo.
- [ADDED] SOMPlots documentation.
- [REMOVED] Python 3.5 support. Now, only 3.6-3.8 are supported.
- [FIXED] Scikit-learn warnings regarding validation of positional arguments.
- [FIXED] Sphinx documentation warnings.

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- [ADDED] Support for Python 3.8.x.
- [ADDED] Test coverage and MultiOutput test.
- [CHANGED] Function `setPlaceholder` to `_set_placeholder`.
- [FIXED] Documentation links